From 61b5e8c0974e29cc4ee2386c456d2dd9af2137e1 Mon Sep 17 00:00:00 2001
From: Manuel Mujica <manuel.expat@gmail.com>
Date: Tue, 27 Feb 2018 13:31:30 -0700
Subject: [PATCH] Pass down previous value to onValue callback

Closes #129
---
 proto-compute.js      |  4 ++--
 proto-compute_test.js | 14 +++++++++++++-
 2 files changed, 15 insertions(+), 3 deletions(-)

diff --git a/proto-compute.js b/proto-compute.js
index 96c690d..7ad3ba0 100644
--- a/proto-compute.js
+++ b/proto-compute.js
@@ -484,8 +484,8 @@ canReflect.assignSymbols(Compute.prototype, {
 	"can.getValue": Compute.prototype.get,
 	"can.valueHasDependencies": hasDependencies,
 	"can.onValue": function onValue(handler, queue) {
-		function translationHandler(ev, newValue) {
-			handler(newValue);
+		function translationHandler(ev, newValue, oldValue) {
+			handler(newValue, oldValue);
 		}
 		singleReference.set(handler, this, translationHandler);
 		//!steal-remove-start
diff --git a/proto-compute_test.js b/proto-compute_test.js
index 4b4747d..6de8efb 100644
--- a/proto-compute_test.js
+++ b/proto-compute_test.js
@@ -411,7 +411,6 @@ QUnit.test("can-reflect setValue", function(){
 	QUnit.equal(a.get(), "A", "compute");
 });
 
-
 QUnit.test("registered symbols", function() {
 	var a = new Compute("a");
 
@@ -430,3 +429,16 @@ QUnit.test("registered symbols", function() {
 	a[canSymbol.for("can.offValue")](handler);
 	a.set("d"); // doesn't trigger handler
 });
+
+QUnit.test("canReflect.onValue should get the previous value", function(assert) {
+	var a = new Compute("a");
+	var done = assert.async();
+
+	canReflect.onValue(a, function(newVal, oldVal) {
+		assert.equal(newVal, "b");
+		assert.equal(oldVal, "a");
+		done();
+	});
+
+	a.set("b");
+});